1. Field of the Invention
The invention relates to an improved method of and apparatus for posttreatment of an exhaust gas from an internal combustion engine.
2. Prior Art
Apparatuses, with which a substance to be mixed with the exhaust gas from an internal combustion engine is sprayed in metered quantities into an exhaust gas line through which exhaust gas flows are known for two different purposes.
First, a urea-water solution (UWS) is sprayed into the exhaust gas from diesel engines, to reduce the proportion of nitrogen oxides NOx in the exhaust gas. The urea in the sprayed-in solution is converted here in the exhaust gas line itself, or in a hydrolysis catalytic converter optionally located in the exhaust gas line, into ammonia NH3, which in a downstream SCR catalytic converter reduces the nitrogen oxides NOx, forming nitrogen N2 and water H2O. The delivery of the urea-water solution is done either with air reinforcement, through a metering pipe protruding into the exhaust gas flow, or by means of a cooled injection valve. In both cases, particularly at low exhaust gas speeds, it cannot be avoided that some of the urea-water solution will meet the wall of the exhaust gas line, which particularly at low exhaust gas or wall temperatures leads to the development of deposits of urea or urea reaction products. In these deposits, urea or urea reaction products are stored, and at a later point they can be undesirably released again, and particularly in a transient mode of operation of the engine, they can be a hindrance to an unproblematic, optimized denitrification process. Moreover, the deposits can increase the flow resistance of the exhaust gas line and thus impair the exhaust gas flow.
Second, for heating exhaust gas from internal combustion engines downstream of the engine, fuel is injected into the exhaust gas, in order to attain the exhaust gas temperatures required for regenerating a downstream NOx, catalytic converter or diesel particle filter (DPF). The fuel here is usually sprayed purposefully onto a portion of the wall of the exhaust gas line, in order to vaporize the fuel faster, because of the contact with the heated wall of the line, than would take place solely by delivery into the hot exhaust gas. However, since the exhaust gas line is more or less severely cooled, among other factors by the relative wind of the motor vehicle, it cannot always be assured that the wall temperature of the exhaust gas line will be above the boiling temperatures of the higher-molecular ingredients of the commercial fuels used.